How do you make a cube with another cube inside it?
Try putting one cube inside the other, and use a Material like this on your outer cube:

Transparency can be a bit of a pain in eevee sometimes, so I opted for this form of shader graph to give you more control. You can change both the BaseColor as well as the Glossy tint separately (to get that slimy look), as well as change the IOR of the Fresnel if you need the slime to seem thicker/thinner. Also because of the way EEVEE handles transparency, the inner cube cannot be transparent as well (unless you are using a single material - the graph for which is substantially more complicated). Because of this, it doesn't matter quite as much what material you give it (I used a copy of the same one for time savings sake - I just disconnected the Transparent and associated Mix Shader).
Lastly, I threw in a Noise Texture as a Displacement to make the outside more &quot;slimy&quot; and bumpy looking as well. Also, pay attention to the Blend Mode highlighted on the right (switching away from opaque is necessary for transparency in EEVEE)